The production of metallic structures having internal cavities by superplastic forming and diffusion bonding (SPF/DB) is well known. For example, Weisert, et al, describe one method for obtaining such structures in U.S. Pat. No. 4,220,276, issued Sept. 2, 1980. In the method described in that patent yttria of a specific particle size is coated on those areas of two titanium sheets where bonding is not desired and other areas of the sheet are not coated with yttria. The sheets are placed in a sandwiched arrangement and are subjected to coordinated temperature-pressure-time duration conditions to produce diffusion bonding of the workpieces other than where they are coated with the yttria. Gas pressure is applied between the contacting surfaces to cause breakthrough in the area coated with yttria and superplastic forming of at least one of the workpieces, causing it to stretch substantially in excess of its original surface area.
The above described technique will produce a number of useful hollow metallic structures. However, this method does not provide good control of the internal diameter of the enclosed cavity in the area where the diffusion bonding takes place. In certain instances such is particularly desirable. For example, if one is making an airfoil having a leading edge and a trailing edge, it is desirable that the leading edge and trailing edge be relatively heavy and be uniform, including having a uniform internal diameter at the enclosed cavity. Yet, it is also desirable that the sides of the airfoil be relatively thin such as is produced by superplastic forming. Indeed, SPF/DB has not been able to produce such structures in the past. Instead, one has had to utilize precision casting which is limiting in that it does not have the abilitv to produce sufficiently thin walls and is subject to having alpha case structuring on its internal surfaces, or to utilize hot diffusion bonding in vacuum which is of limited usefulness because of restrictions to internal airflow or to utilize conventional fabrication methods which are costly, result in undesirable joints and provide undesirably heavy airfoil structures.